Ductility promoter and stabilizer for electroless copper plating baths

ABSTRACT

The present invention provides a means for controlling the stability of electroless copper plating baths and enhancing the ductility properties of the electroless metal deposits produced therefrom.

United States Patent [72] Inventor [21 Appl. No. [22] Filed [45] Patented [73] Assignee Fredrick W. Schneble, Jr. Oyster Bay, N.Y.

Feb. 4, 1969 Sept. 2 1, 1971 Photocircuits Corporation Glen Cove, N.Y.

[54] DUCTILITY PROMOTER AND STABILIZER FOR ELECTROLESS COPPER PLATING BATHS 13 Claims, No Drawings [50] Field of Search 106/1; 117/130,130 E, 160

[56] References Cited UNITED STATES PATENTS 2,472,393 6/1949 Avallone et a1 106/1 X 3,095,309 6/1963 Zeblisky et a1. 106/1 3,246,995 4/1966 Moore 106/1 3,472,664 10/1969 Bastenbeck et a1 106/1 Primary Examiner-Lorenzo B. Hayes Attorney-Morgan, Finnegan, Durham & Pine deposits produced therefrom.

DUCTlLlTY PROMOTER AND STABILIZER FOR ELECTROLESS COPPER PLATING BATHS The present invention relates to electroless plating of metals, and more particularly to controlling the stability of electroless metal plating baths and enhancing the physical properties of the electroless metal deposits produced therefrom.

The present invention has for an object to improve the stability of electroless copper baths without adversely affecting the deposition rate or the physical properties of the electroless copper produced therefrom.

Another object of this invention is to provide means for monitoring electroless copper solutions so as to maintain them in a state of dynamic equilibrium.

A further object of the invention is to provide electroless copper deposition solutions which are capable of producing electroless copper having enhanced physical properties including improved ductility, brightness, and the like.

Still a further object of this invention is to provide new and useful addition agents for controlling the stability of electroless copper solutions.

The present invention and the agents described herein although generally applicable to electroless metal plating solutions, are particularly useful with electroless copper solutions.

lilcctroless copper solutions are capable of depositing copper without the assistance of an external supply of electrons. Typically, such solutions comprise water, a small amount of copper ions, a complexing agent for copper ions, and a pH regulator.

The selection of the water soluble copper salt for such baths is chiefly a matter of economics. Copper sulfate is preferred for economic reasons, but the halides, nitrates, acetates and other organic and inorganic acid salts of copper may also be used.

Rochelle salts, the sodium salts (mono-, di-, tri-, and tetrasodium), salts of ethylenediaminetetraacetic acid, nitrilotriacetic acid and its alkali salts, gluconic acid, gluconates, and triethanolamine are preferred as copper ion complexing agents, but commercially available glucono-9-loctone and modified ethlenediamineacetates are also useful, and in certain instances, give even better results than the pure sodium ethylenediamine-tetraacetates. Such materials are N- hydroxyethylethylenediamine-triacetate and N, N, N, N tetrakis (Z-hydroxypropyl ethylenediamine). Other materials suitable for use as cupric complexing agents are disclosed in U.S. Pat. Nos. 2,938,805; 2,996,408, 3,075,855 and 3,075,856.

Copper reducing agents which have been used in electroless metal baths include formaldehyde, and formaldehyde precursors or derivatives, such as paraformaldehyde, trioxane, dimcthyl hydantoin, glyoxal, and the like. Also suitable as reducing agents in alkaline baths are borohydrides, such alkali metal borohydrides, e.g., sodium and potassium borohydride, as well as substituted borohydrides, e.g., sodium trimethoxyborohydride. As reducing agents in such baths may also be used boranes, such as amine borane, e.g., isopropylamine borane, morpholine borane, and the like.

Typical of the copper reducing agents for use in acid electroless copper solutions are hypophosphites, such as sodium and potassium hypophosphite, and the like.

The pH adjuster or regulator may consist of any acid or base, and here again the selection will depend primarily on economics. For this reason, the pH adjuster on the alkaline side will ordinarily be sodium hydroxide. On the acid side, pH will usually be adjusted with an acid having a common anion with the copper salt. Since the preferred copper salt is the sulfate, the preferred pH adjuster on the acid side is sulfuric acid.

In addition to the materials placed in the copper bath solution, as disclosed above, there may be materials placed in the bath to further enhance the properties of the solution. These may include cyanide compounds such as sodium and potassium cyanide and nitriles such as acrylonitrile; membered heterocyclics such as thiazoles and iso-thiazoles, e.g., 2-mercaptobenzolthiazole and the like; and sulfurated potash. Other materials suitable for stabilizing electroless copper bath solutions are disclosed in U.S. Pat. Nos. 3,095,309, 3,257,2 l5, and 3,361,580.

in operation of the bath, the copper salt serves as a source of copper ions, and the reducing agent reduces the copper ions to metallic form. The reducing agent is itself oxidized to provide electrons for the reduction of the copper ions. The complexing agent serves to complex the copper ion so that it will not be precipitated, e.g., by hydroxyl ions and the like, and at the same time makes the copper available as needed to the reducing action of the reducing agent. The pH adjuster serves chiefly to regulate the internal plating potential of the bath.

The performance of the electroless copper baths is improved by addition thereto of certain surfactants in an amount of less than about 5 grams per liter. Such surfactants include organic phosphate esters and oxyethylated sodium salts, and mixtures thereof. Preferred surfactants are alkylphenoxy polyethoxy phosphate esters. Such surfactants may be obtained under the trade name of Gafec RE 610 It should be understood, however, that every constitutent in the electroless copper bath has an eflfect on plating potential, and therefore must be regulated in concentration to maintain the most desirable plating potential for the particular ingredients and conditions of operation. Other factors which affect internal plating voltage, deposition quality and rate include temperature and degree of agitation, in addition to type and concentration of the basic ingredients mentioned.

In electroless plating baths, the bath constituents are continuously being consumed, so that the bath is in a constant state of change. Control of such baths, so as to maintain a relatively high plating rate over relatively long periods of time is exceedingly difficult. As a result, such baths, and particularly those having a high plating potential, i.e., highly active baths, tend to become unstable and to spontaneously decompose with use. Heretofore, spontaneous decomposition of high plating potential baths has been an important factor in limiting the commercial acceptance of electroless copper solutions as a substitute for or a competitor of electroplating baths.

According to the present invention, it has been discovered that certain agents, when added to electroless copper plating solutions, serve to maintain the baths in a dynamic state of equilibrium for long periods of time and to prevent spontaneous decomposition.

The addition agents of this invention render electroless copper solutions less sensitive to changes of temperature and concentration, and therefore permit greater variation in operating conditions, ingredient concentration, temperature, and types ofingredients than have heretofore been considered possible.

The stabilizing agents of this invention are simple or complex compounds of polyalkylene oxides such as polyethylene polypropylene, and polybutylene. Tlhe polyalkylene oxides containing less than five to seven carbon atoms are preferably included as the stabilizing agents. Although all these polyalkylene oxides are applicable in the present invention, the polyethylene oxides are preferable and have shown advantageous results when used in various electroless plating solutions.

The polyethylene oxides according to this invention are nonionic water soluble homopolyrner resins having high molecular weights ranging from several hundred thousands to five million and above. They are thermoplastics which are completely soluble in water and compatible in moderate concentrations of electrolytes.

The polyethylene oxides suitable for use in the present invention are sold by the Union Carbide Corporation under the trade name of POLYOX resins. The water soluble resins are manufactured in different grades of varying molecular weights. The preferred grade being that produced under the trade name of POLYOX coagulant, having a molecular weight of greater than 5,000,000.

The amount of polyethylene oxide maintained in the baths will be a small effective amount. Ordinarily, its concentration will average between about 0.4 and 80.0 milligrams per liter of solution. preferably about l.0 milligrams per liter.

It should be emphasized however that the small effective amount of polyethylene oxide will vary with the nature of the particular compound used, and with makeup of the solution and the conditions e.g. temperature, under which it is used.

it is noted that the stabilizing agents of the present invention, e.g., polyethylene oxides, may be added to electroless plating solutions containing known stabilizers as sodium cyanide and 2-mercapto-benzathiazole, to greatly enhance the stability of such plating solutions. With a small amount of a polyethylene oxide added to the plating solutions containing these known stabilizers, the stability of the plating solutions is greatly increased.

in addition to stabilizing the bath, the polyethylene oxides enhance the physical properties of the electroless copper deposits, particularly the ductility. This is a completely surprising result and contributes materially to the value of the baths utilizing the instant invention.

Overall, the higher molecular weight polyethylene oxides provide greater stability of the plating baths and enhance the ductility of the copper deposited from the plating solutions of this invention. For example, a bath having a polyethylene oxide of a molecular weight of 100,000 has greater stability and provides a higher ductility of copper deposits than a comparable bath having a polyethylene oxide of a molecular weight of 6,000. The optimum range of molecular weight of the polyethylene oxides for enhanced ductility is between 100,000 and 6,000,000.

It has also been found that the rate of deposition of the metal from the plating baths using polyethylene oxides is increased with the use of compounds having high molecular weights ranging over 1,000,000.

Typical electroless copper deposition baths made according to the present invention are as shown below. A polyethylene oxide having a molecular weight of greater than 5,000,000 is used in each bath.

Bath 1 Copper salt50 mmoles/liter.

Reducing agent80 mmoles/liter.

Copper ion complexing agent-70 rnrnoles/liter.

Stabilizing agent (as polyethylene oxide)0.l to 1.0

mg./liter.

Na. CN15.0 rug/liter.

Sulfurated potash0.3 mg./liter.

WaterSuflicient to make one liter.

Temperature45 C.

Bath 2 Copper Salt50 mmoles/liter.

Reducing agent-80 mmoles/liter.

Copper ion complexing agent-70 mrnoles/liter.

Stabilizing agent (as polyethylene oxide)0.1 to 1.0

In ./liter.

Na N25.0 rug/liter.

2-rnercapto-benzothiazolc0.1 1ng./liter.

Surfactant (Gafae 610)0.25 g./liter.

Temperature60 C.

Bath 3 Copper salt-50 mmoles/liter. Reducing agent--80 Inrnoles/liter. Copper ion complexing agent70 mmoles/liter. Stabilizing agent (as polyethylene oxide)10.0 mg./liter. Surfactant (Gafac 610)0.25 g./liter. pH12.2.

Temperature60 C.

in considering the general and specific working formulas set forth herein, it should be understood that as the baths are used up in plating, the ingredients will be replenished from time to time. Also, it is advisable to monitor the pH. and the concentration of the additive clement described, and to adjust them to their optimum value as the bath is used.

The baths may be used at widely varying temperatures. c.g., between l5 and l00 C., although they will usually be used between about 20 and C. As the temperature is increased, but the temperature is not highly critical and, within the usual operating range, excellent bright, ductile deposits of copper are obtained.

Bath 3 varies from the other two baths in that it contains one stabilizing agent, polyethylene oxide, the agent provided according to the present invention. Bath 3 was very stable and the copper deposited from the bath had a high ductility.

Performance data for the Baths 1 and 2 made in accordance with the teachings contained herein are given in tables I and I1, respectively. The tables show the results with respect to the rate of copper deposition, ductility of deposition and stability of baths prepared according to formulas set forth herein.

TABLE 1.(BATH 1) Rate of deposition Ductility Polyethylene oxide (mg./liter) (mg./em. /hr.) (Bends) Stable 1. 2 0 No. 1. 4 0. 5 Yes. 1. l 0. 5 Yes. 1. 18 1. 5 Yes. 1. 18 2. 5 Yes.

TABLE II-(BATH 2) Deposition Polyentylene oxide (mg/liter) (mg./cm. hr.) Ductility Stable 1.3 2. 5 No. 1. 2 4. 0 Yes. 1. 2 4. 6 Yes. 1. 2 5. 0 Yes. 1. 2 6. 0 Yes. 1. 2 5. 5 Yes.

in each of the baths, both a printed circuit plate and a ductility sample were placed for a period of about 5 hours.

In the tables, the ductility is measured by bending the copper deposit through in one direction, creasing, then returning it to its original position, with pressing along the crease to flatten it, this cycle constituting one bend.

Use of the polyethylene oxide in copper solutions improves and maintains the stability as is brought out in tables I and ii. As indicated in the tables, when the polyethylene oxide was added to the baths the plating solutions became stable. A plating solution is stable when no powder is formed on the plated specimen. Another showing of stability of a plating solution is when there are no deposits of metal on the walls or bottom of the bath container in 24 hours. Conversely, if there are deposits of metal on the walls and at the bottom of the plating bath container in less than 24 hours, the solution is not stable.

As also shown by the tables I and ii, the presence of polyethylene oxide also enhances the ductility of the copper deposits.

in using the electroless copper solutions to plate metal, the surface to be plated must be free of grease and other contaminating material.

Where a nonmetallic surface is to be plated, the surface area to receive the deposit must first be sensitized to render it catalytic to the reception of electroless copper, as by the well known treatment with an acidic aqueous solution of stannous chloride (SN C1 followed by treatment with a dilute aqueous acidic solution of palladium chloride (Pd C1 Alternatively, extremely good sensitization of nonmetallic surfaces is achieved by contact with an acidic solution containing a mixture of stannous chloride and precious metal chloride, such as palladium chloride, the stannous chloride being present in stoichiometric excess, based on the amount of precious metal chloride.

Where a metal surface is to be plated, it should be degreased, and then treated with an acid, such as hydrochloric or phosphoric acid, to free the surface of oxides.

Following pretreatment and/or sensitization, the surface to be plated is immersed in the electroless copper baths, and permitted to remain in the bath until a copper deposit of the desired thickness has been built up.

The invention in its broader aspects is not limited to the specific steps, processes and compositions shown and described, but departures may be made therefrom within the scope of the accompanying claims without sacrificing its chief advantages.

lclaim:

1. In an electroless copper deposition solution which comprises water, a complexing agent for copper ion to be deposited, a reducing agent for the copper ion to be deposited, a stabilizing agent for the copper ion to be deposited, and an agent capable of adjusting pH, the improvement which comprises maintaining in the solution a small effective stabilizing amount of at least about 0.1 milligram per liter of a polyalkylene oxide, having up to about 7 carbon atoms per alkylene moiety and a molecular weight of at least about 6000.

2. The solution of claim 1, wherein the content of said polyalkylene oxide is between about 0.4 and 80.0 milligrams per liter, said amount being insufficient to prevent electroless deposition of copper but sufficient to enhance the stability of the solution at said conditions.

3. The solution of claim 1, wherein the polyalkylene oxide content is between about 1 and 10 milligrams per liter.

4. The solution of claim 1, wherein the polyalkylene oxide has a molecular weight ranging from 6,000 to greater than 5,000,000.

5. The solution of claim 1, wherein the polyalkylene oxide is a member selected from the group consisting of polyethylene oxide, polypropylene oxide and polybutylene oxide.

6. The solution of claim 1, wherein the pH adjuster maintains the pH in the alkaline range.

7. The solution of claim 1, wherein the stabilizing agent is sodium cyanide.

8. The solution of Claim 1, wherein the reducing agent for the copper ion is formaldehyde.

9. In a process for depositing copper on a catalytic surface comprising contacting said surface with an electroless copper solution comprising a water soluble copper salt, a complexing agent for a copper ion to be deposited, a reducing agent for the copper ion to be deposited, a stabilizing agent for the copper ion to be deposited, and an agent for adjusting pH, the improvement which comprises maintaining in the solution a small effective stabilizing amount of at least about 0.1 milligrams per liter of a polyalkylene oxide, having up to about seven carbon atoms per alkylene moiety and a molecular weight of at least about 6000.

10. The process of claim 9, wherein the polyalkylene oxide is present in an amount of between about 0.4 and 80.0 milligrams per liter, said amount being insufiicient to prevent electroless deposition of copper but sufficient to enhance the stability of the solution at said conditions.

11. The process of claim 9, wherein the polyalkylene oxide is a member selected from the group consisting of polyethylene oxide, polypropylene oxide and polybutylene oxide.

12. The process of claim 9, wherein the polyalkylene oxide is present in an amount of between about 1.0 and 10.0 milligrams per liter.

13. The process of claim 9, wherein the polyalkylene oxide has a molecular weight ranging from 6,000 to greater than 5,000,000. 

2. The solution of claim 1, wherein the content of said polyalkylene oxide is between about 0.4 and 80.0 milligrams per liter, said amount being insufficient to prevent electroless deposition of copper but sufficieNt to enhance the stability of the solution at said conditions.
 3. The solution of claim 1, wherein the polyalkylene oxide content is between about 1 and 10 milligrams per liter.
 4. The solution of claim 1, wherein the polyalkylene oxide has a molecular weight ranging from 6,000 to greater than 5,000,000.
 5. The solution of claim 1, wherein the polyalkylene oxide is a member selected from the group consisting of polyethylene oxide, polypropylene oxide and polybutylene oxide.
 6. The solution of claim 1, wherein the pH adjuster maintains the pH in the alkaline range.
 7. The solution of claim 1, wherein the stabilizing agent is sodium cyanide.
 8. The solution of Claim 1, wherein the reducing agent for the copper ion is formaldehyde.
 9. In a process for depositing copper on a catalytic surface comprising contacting said surface with an electroless copper solution comprising a water soluble copper salt, a complexing agent for a copper ion to be deposited, a reducing agent for the copper ion to be deposited, a stabilizing agent for the copper ion to be deposited, and an agent for adjusting pH, the improvement which comprises maintaining in the solution a small effective stabilizing amount of at least about 0.1 milligrams per liter of a polyalkylene oxide, having up to about seven carbon atoms per alkylene moiety and a molecular weight of at least about
 6000. 10. The process of claim 9, wherein the polyalkylene oxide is present in an amount of between about 0.4 and 80.0 milligrams per liter, said amount being insufficient to prevent electroless deposition of copper but sufficient to enhance the stability of the solution at said conditions.
 11. The process of claim 9, wherein the polyalkylene oxide is a member selected from the group consisting of polyethylene oxide, polypropylene oxide and polybutylene oxide.
 12. The process of claim 9, wherein the polyalkylene oxide is present in an amount of between about 1.0 and 10.0 milligrams per liter.
 13. The process of claim 9, wherein the polyalkylene oxide has a molecular weight ranging from 6,000 to greater than 5,000,000. 